new driver: kubernetes

Tested with `kind` and GKE.

Note: "nodes" shown in `docker buildx ls` are unrelated to Kubernetes "nodes".
Probably buildx should come up with an alternative term.

Usage:

  $ kind create cluster
  $ export KUBECONFIG="$(kind get kubeconfig-path --name="kind")"

  $ docker buildx create --driver kubernetes --driver-opt replicas=3 --use
  $ docker buildx build -t foo --load .

`--load` loads the image into the local Docker.

Driver opts:

  - `image=IMAGE` - Sets the container image to be used for running buildkit.
  - `namespace=NS` - Sets the Kubernetes namespace. Defaults to the current namespace.
  - `replicas=N` - Sets the number of `Pod` replicas. Defaults to 1.
  - `rootless=(true|false)` - Run the container as a non-root user without `securityContext.privileged`. Defaults to false.
  - `loadbalance=(sticky|random)` - Load-balancing strategy. If set to "sticky", the pod is chosen using the hash of the context path. Defaults to "sticky"

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

vendor/github.com/golang/protobuf/proto/equal.go

@@ -246,7 +246,8 @@ func equalExtMap(base reflect.Type, em1, em2 map[int32]Extension) bool {
 			return false
 		}
 
-		m1, m2 := e1.value, e2.value
+		m1 := extensionAsLegacyType(e1.value)
+		m2 := extensionAsLegacyType(e2.value)
 
 		if m1 == nil && m2 == nil {
 			// Both have only encoded form.